In conventional systems, a single offset image record is usually used which, however, is regularly updated. The radiography mode differs from the fluoroscopy in that the irradiation periods can last several seconds and preferably individual images are recorded whereas in the fluoroscopy mode, a multiplicity of images is recorded at high frame rate with short irradiation times. In the radiography mode, too, several images are recorded successively, however. Between the recording of individual frames, it is then frequently not possible to update the offset image record. To update the offset image record, offset images must be recorded. There are so-called dark images. In the absence of X-ray irradiation, the flat-panel X-ray detector records image data as if an X-ray image were recorded. These offset images (dark images) then display the pixel-by-pixel offset generated by the detector elements of the flat-panel X-ray detector. During the recording of offset images, the image recording period should be exactly as long as in the case of the associated X-ray images to be corrected (bright images). In the case of image recording periods of several seconds, the radiologist recording the images can feel disturbed if an offset image is currently recorded and the radiography X-ray image cannot be recorded for a period of some seconds. In conventional systems, therefore, clever strategies have been developed of how and when the offset image record is updated.
For a purely radiographic system, an offset strategy is described in US 2005/0063511 A1. There are similar offset strategies also for fluoroscopic systems. More recent flat-panel X-ray detectors for universal applications allow the alternate production of a radiography mode and a fluoroscopy mode. Previous approaches involve the use of the offset strategies developed for the purely radiographic and purely fluoroscopic systems.
All previous strategies have in common that the offset image record is recorded before the X-ray image record to be corrected in each case in time. The reason given for this in conventional systems is that flat-panel X-ray detectors frequently exhibit so-called ghosting so that the ghost is in effect after the recording of an X-ray image. Since the offset image record is always used for a number of X-ray image records or correspondingly, offset image recordings are used for updating an offset image record which is used for a number of such X-ray image records, the ghost has a disturbing effect. In radiography, ghosts are effective for up to 15 seconds so that the offset images have hitherto been always recorded before the associated X-ray images for reasons of timing in conventional systems in the offset update strategies.